If I had a firewired iMac with a 800FW port and cable, couldn't I plug this TH/FW connector to the external HD and then plug the FW cable to the female FW port? In other words, using the TH/FW connector with a 800FW cable.

No. It's for connecting FireWire peripherals to a Thunderbolt port. It doesn't work the other way round. Also there would be little benefit in it working the other way round as the FireWire port on your iMAC would be a bottleneck so you would see no noticeable speed increase over a standard FireWire drive.